Structure and spectra of H2O in hydrated β-alumina

Abstract

The structure and spectra of hydrated Li and Na β-alumina were investigated using neutron diffraction, infrared absorption, and Raman scattering. The dimensions of the hexagonal unit cell of a hydrated Li β-alumina crystal containing 1.55 H2O molecules per unit cell are a=5.591 Å and c=22.715 Å. The oxygen atoms of the water molecules are located in the conduction plane between the mO, and the aBR sites; the protons, located above the below the plane, form bent hydrogen bonds with the O(4) oxygen ions. The HOH bond angle of water in Li β-alumina is 114° and the ‖O–H‖ bond distance is 0.992 Å. Based on polarized infrared spectra, H2O adopts a similar structure and orientation in Na β-alumina. Spectra of absorbed H2O, D2O, and HDO species show that water molecules dissociate in Li β-alumina to form OH− and H(H2O)+n species. No evidence was found for the dissociation of water in Na β-alumina. The absorption coefficients determined for OH− and H2O in Li β-alumina include local field corrections. A large local field anisotropy at the protons of H2O is responsible for the large ratio of the intensities of ν3 and ν1 observed for water in Li and Na β-alumina.